The present invention is in the field of direct solar-to-electrical energy conversion using refrigerated photovoltaic cells subjected to concentrated solar energy or sunlight.
The optimal design of conventional cylindrical solar concentrators which focus all the solar energy incident on its cylindrical collector surface of arbitrary cross-section requires that the angle formed by the sun ray projection on the plane normal to the cylinder generatrices and the axis of the concentrator be smaller than what is known as the acceptance angle .epsilon.m of the concentrator.
For such optimal concentrators, the relationship EQU G=1/sin .epsilon.m
is verified, where the gain G is the ratio between the concentrator entrance aperture length and the collector perimeter. The relationship between G and .epsilon.m constitutes a thermodynamic limit in the trade off between both parameters if only a homogeneous means is transversed by the sun's rays.
Optical concentrators of, for example, the cylindrical type, have been used in connection with planar solar cells by placing the negative contact area (lower surface) of the cells adjacent the bottom of the concentrator wherein only one side (upper surface) of each solar cell is exposed to concentrated solar radiation, resulting in only one side of the cell being photovoltaically active. Conventionally, the unexposed solar cell lower surface is utilized to transfer undesirable heat built-up in the cell to thermal radiators or water forced refrigeration means.
Hollow pipes of single crystal silicon made by a well-known technique, with a solar cell integrally formed on the outer surface of the pipe so as to wrap-around it, have also been placed in cylindrical solar concentrators. In this case, forced circulation of water through the interior of the pipe removes the undesirable heat from the underside of the wrapped-around cell.
A large acceptance angle .epsilon.m is desired in cylindrical concentrators since it allows the concentrator to remain static, thus precluding the need for sun tracking type moving parts. Only periodic seasonal changes of orientation are necessary.
It would be desirable to provide planar solar cells having both sides photovoltaically active in a cylindrical optical concentrator since such a structure would allow maximum utilization of the high cost semiconductor mass for a given acceptance angle .epsilon.m.
A major problem encountered in attempting to use planar solar cells having two photovoltaically active sides lies in the fact that heat generated in the cell when it is used in an solar energy concentrator must be removed.
The planar solar cells of the prior art have only one photovoltaically active side. A multijunction cell formed by a technique developed long ago in the manufacturing of high voltage diodes (used in T.V. sets) is related to a two-sided solar cell. The technique includes the steps of piling up a set of silicon wafers in which a pn junction has been previously formed; placing sheets of aluminum foil between next adjacent silicon wafers; heating the stack to alloy the aluminum to the silicon wafers; and sawing the stack along lines parallel to its longitudinal axis to form slices which present pn diodes connected in series. The characteristic of this multijunction type of cell is to provide low current and high voltage. Due to the low current characteristic of this cell, it is suitable for applications where voltage drops due to high currents are to be avoided.
While the multijunction cell is inherently a double-sided cell, it has never been used in the environment of double-sided concentrated solar illumination wherein cell cooling must be provided. Further, lateral illumination of the multijunction cell would be very inefficient so as to require large amounts of concentrated solar illumination that cannot be provided by known optical concentrators. Finally, the lateral sawing process step in forming multijunction devices wastes high cost silicon.